Consider two firms with the following marginal abatement costs (MAC) as a function of emissions (E):

MAC_1 = 4 - 2E_1

MAC_2 = 8 - E_2,

and assume marginal external damages (MED) from aggregate emissions from the two firms (E_Agg) is:

MED = E_Agg.

The total external damages at the socially efficient level of aggregate emissions is ____.